1. Field of the Invention
The present invention relates to a torque coupling device provided between two relatively rotatable shafts for transmitting a rotational torque or power therebetween. More particularly, it relates a torque coupling device of the type designed for use, for example, between front and rear road wheel axles of a four-wheeled vehicle.
2. Discussion of the Prior Art
In general, a coupling device for transmitting a torque between front and rear axles of a four-wheeled vehicle is provided with a multiple-disc clutch for transmitting the torque between the front and rear axles by the action of friction engagement of the clutch. In the coupling device of this kind, it may occur that a multiple-disc clutch is damaged because of being overheated when frictional sliding due to incomplete connection is continued for a long period of time for the vehicle to escape from a stuck situation.
Another type of torque coupling devices for the four-wheeled vehicles comprises a torque transmitting multiple disc clutch, a piston for actuating the clutch and pressure generating means for generating a pressure depending on the difference in rotational speed between front and rear road wheels and for applying the pressure to the piston so as to bring the multiple disc clutch into a friction engagement state. In the coupling device of this latter type, the transmissive torque varies as indicated by the solid line in FIG. 7 in dependance upon the differential rotational speed and tends to become saturated within the range of the rotational speed difference being large. This may disadvantageously results in that the multiple disc clutch is damaged because of being overheated when friction sliding due to incomplete connection is continued for a long period of time for the vehicle to escape from a stuck state.
To solve the problems, a torque coupling or transmitting device has been proposed, which as described in Japanese Unexamined Patent Publication No. 1-93631, is provided with a lock-up mechanism utilizing a cam. However, the known device accompanies drawbacks that it becomes large in size due to the incorporation of the lock-up mechanism thereinto and also results in a high manufacturing cost due to increases in number of the parts therefor.